Prior to a June, 1984 FCC decision, pay telephones were the exclusive province of the various Telephone Companies while others were precluded from the business of providing pay telephone service. Today, however, subject to state Public Utility Commission regulations, Customer Owned Coin Telephone (COCT) service is permitted, but a number of non-trivial technical challenges have been created including called party answer detection.
Coin telephones (pay phones) owned by the local Telephone Company generally utilize DC signals to signal called party answer. Such information is specially communicated between telephone central offices and then to the originating pay phone telling it when to accept the deposited coins. Such information, however, is not communicated to conventional telephones and it is only with great difficulty that called party answer can be detected. Naturally, the calling party knows when a connection is completed; but a knowledge of human frailty suggests that he not be relied upon to turn the "meter" on.
Complicating the problem of answer detection is the existence of special information messages that are provided when, for example, a telephone has been disconnected or a new telephone number has been assigned; and while certain tones are indicative of a proper answer, other tones are not. Ringing, busy and reorder signals as well as background noise and feedback from the calling party herself (sidetone) make answer detection a formidable challenge.
U.S. Pat. No. 4,122,308 suggests that a called party answer can be sensed by one or more of the following techniques: (i) detecting a line reversal signal, if available, (ii) detecting the presence of a voice on the telephone line, or (iii) sensing that a time has elapsed since the last ring which is greater than the time between rings and that no busy signal or dial tone has occurred. While each of these techniques has merit in general, when specifically applied to each of the various signal conditions potentially present on the telephone line, they do not offer sufficient accuracy, without more, to warrant use.
Whereas overcharging incurs the wrath of the calling party, undercharging angers the owner of the pay phone who ultimately pays for the call. Clearly there is little room for error in this field, especially in view of the history of accurate charging established by existing pay phones.
It is therefore an object of the present invention to provide accurate answer detection for pay phones in the absence of special signals from the local Telephone Company and without relying on the calling party to so indicate.